GOOD CONSTITUTIONAL law can on occasion lay the groundwork for bad public policy. That seems to be the potential, anyway, of the Supreme Court's decision the other day in a suburban zoning case. The Court said that a community can refuse to change zoning rules that have the effect of discriminating against minorities - so long as it does so for other than racial reasons. That, it seems to us, is good constitutional law: logical, understandable, defensible. But its impact may be something else again.

The Court's decision makes absolutely clear the principle on which it has been operating for some time: The ultimate test of whether an official act denies minority-group citizens the equal protection of the law is not whether it affects those people in some particular way, but, rather, how it was intended to affect them. The difference between these two standards is enormous and can produce starkly different judgments of what a community must do to fulfill its obligations under civil rights law. By refusing to rezone a piece of land from single-family to multiple-family use, an almost all-white suburb of Chicago has succeeded so far in keeping out low and moderate-income housing projects that would have brought many black residents. Testing this refusal by its racial effect, a lower court had found the denial of rezoning discriminatory and unconstitutional. Testing it by its intent, the Supreme Court, on the other hand, found it constitutional. It took note of the fact that the community had a history of not down-zoning this kind of property, the purpose being to maintain its village plan and protect property values.

Plainly, the Court will use the same logic in other areas of discrimination. And, this is likely to make it more difficult to challenge official acts as discriminatory on any groups. For it is much easier to prove that an action produces or preserves racially discriminatory conditions than to prove it ws intended to do so.

In fact, the task has been made more difficult still. For in a footnote to its decision, the Court pointed out that even an action taken for the purpose of discriminating is valid when the government can show that the same action would have resulted if the illegal purpose had not been considered.

Not even the purest of the pure among constitutionalists, however, would dispute the fact that sometimes laws or rules adopted with the best of motives need to be changed simply because they operate to put minorities at an unfair disadvantage. The effect of the Court's ruling is probably going to be this: Those changes will henceforth have to be sought outside the courts - in legislatures and on the ballot. In the long run, those are not bad places for this particular public business to be undertaken. Once accepted via the political process - as was the Fair Housing Act, by the way, which may yet get this particular project built - efforts to ensure a more equitable society take on a force and vitality and meaning that no federal court can ever give them.